Hybrid dysgenesis, a recently discovered phenomenon in Drosophila melanogaster, is a syndrome of associated aberrant traits which occurs non-reciprocally in the F1 hybrid progeny of crosses between certain strains. The syndrome includes mutation, sterility, male recombination, distorted transmission ratios, chromosomal aberrations, non-disjunction and some local increases in female recombination. Complex interstrain interactions are involved in the etiology of hybrid dysgenesis and environmental factors, such as temperature, also have a critical effect on its manifestation. Several studies suggest that the potential for hybrid dysgenesis is geographically widespread in both natural and laboratory populations. The objective of the proposed research is to investigate the etiology and genetic nature of hybrid dysgenesis. Emphasis will be placed on the causes of the non-reciprocal properties and the apparent limitation to interstrain hybrids. The objective will be approached in a series of laboratory experiments using standard Drosophila techniques. The Drosophila strains to be used have undergone extensive preliminary testing in this laboratory.